The present invention relates to an improvement in an activated sludge treatment apparatus and particularly to the separation of activated sludge and treated water from each other.
Some related activated sludge treatment facilities are composed principally of a sludge tank 1 and a sedimentation basin 2, as illustrated in FIG. 2. The treatment of waste water using such type of activated sludge treatment facilities is performed as follows. First, waste water is introduced into the sludge vessel 1 and contacted with activated sludge under the supply of air to allow microbial oxidation to proceed. Then, effluent from the sludge tank 1 is fed to the sedimentation basin 2 to allow natural sedimentation to take place. Supernatant liquid in the sedimentation basin 2 is discharged as treated water through, for example, a chlorine sterilization basin, while activated sludge precipitated in the sedimentation basin 2 is returned to the sludge tank 1.
In actual facilities, pre-treatment is conducted in many cases using, for example, a sedimentation basin (a first sedimentation basin) provided before the sludge tank 1. Activated sludge in an amount corresponding to an increase in the sedimentation basin 2 is fed as excess sludge to a sludge disposal apparatus (not shown).
The above related activated sludge process is disadvantageous in that the natural sedimentation in the sedimentation basin 2 requires a considerable time and that a large capacity is required for the sedimentation basin 2. Further, when attention is paid to the sludge concentration (2,000-4,000 ppm) in the sludge tank 1, the higher the sludge concentration, the larger the treating capacity, but a considerable time is required for the natural sedimentation.
In order to eliminate the above-mentioned drawbacks it has been proposed to use a high polymer membrane such as an ultrafilter in place of the sedimentation basin 2. The use of an ultrafilter is advantageous in that a considerable time for the natural sedimentation is no longer required, nor is such a large capacity as the sedimentation basin 2 required.
In such high polymer membrane, however, care must be exercised in scratching off the deposited on the membrane surface and in washing. This is because breakage of the membrane would make its reuse impossible. Thus, ultrafilters and other high polymer membranes involve problems in point of durability.